Hundreds of birds, some endangered, have been killed by a botulism outbreak at the Miranda wetland south of Auckland.

A major cleanup operation is underway on the shores of the Firth of Thames as Fish and Game, DOC and Miranda Shorebird Centre workers remove the dead birds.

Miranda is an internationally significant wetland home to thousands of waterfowl and shorebirds, it's also a stop off point for migrating godwits.

Fish and Game Auckland Waikato Manager David Klee said the situation is grim.

He said some of the birds have died slow and agonising deaths.